Animals such as mammals (including humans) are often susceptible to parasite infections and infestations. These parasites may be ectoparasites, such as insects, and endoparasites such as filariae and other worms. Production animals, such as cows, pigs, sheep and goats, can be infected with one or more trematodes. Of particular concern here is Fasciola hepatica (i.e., liver fluke or F. hepatica).
Liver flukes are a particular problem because they adversely affect the health of the animal or human and can inflict significant economic loss in a domestic livestock population. It is estimated that F. hepatica poses a risk to at least 250 million sheep and 350 million cattle worldwide. Moreover, domestic animals other than sheep and cows may serve as intermediate hosts. Liver flukes can cause liver condemnation, secondary infections, reduced milk and meat production, abortion and fertility problems.
Several types of control measures for liver flukes have been introduced over the past century. First, halogenated hydrocarbons (e.g., CCl4−; carbon tetrachloride) were introduced for ruminants in the 1920s. Halogenated hydrocarbons had limited success and are no longer used primarily because of their adverse effects and variable efficacy. Second, halogenated phenols were administered in the late 1950s (e.g., hexachlorophene and bithionol sulfoxide) followed by the similar halogenated salicylanilides (e.g., oxyclozanide, bromoxanide). Fourth, benzimidazole carbamates (e.g., albendazole, luxabendazole) were found to have a broad anthelmintic spectrum against nematodes and mature F. hepatica. Another benzimidazole—the chlorinated methylthiobenzimidazole derivative triclabendazole—has a high success rate against F. hepatica. Fifth, bisanilino compounds introduced in the 1960s were intolerable due to toxic side effects. Finally, benzene sulfonamides (e.g., clorsulon) were studied in the 1970s. Extensively modified examples of this class demonstrate high efficacy on both mature and immature F. hepatica. Of these six classes of anthelmintics the benzimidazole class is perhaps the most widely used for its high efficacy.
The benzimidazole anthelmintics are widely used to treat internal worm parasites. U.S. Pat. No. 4,197,307 discloses 6-phenyl substituted benzimidazoles useful for treating trematodes. The '307 patent discloses a substitution from the sulfur atom at the 2-position of the imidazole ring as well as a substituted aryloxy or thioaryl group from the 6-position of the benzene ring.
U.S. Pat. No. 4,205,077 discloses benzimidazole sulfides as anthelmintic agents. While claiming the same basic 6-phenyl substituted structure of the '307 patent, the '077 patent differs in that the sulfur at the 2-position of the imidazole ring is not substituted, leaving it available to form a dimer linked by a disulfide bond.
U.S. Pat. No. 4,336,262 discloses a pour-on anthelmintic that is heavily substituted at the 7-position of the benzimidazole ring. In particular, the substitution is a sulfamoyl moiety while the 5- and 6-positions are minimally substituted.
U.S. Pat. No. 4,468,390 discloses an anthelmintic composition that is a mixture of a macrolide antibiotic and one of a benzimidazole, a salicylamide or an isoquinoline compound. The benzimidazole compounds disclosed as suitable for use in the '390 patent are 2-(methoxycarbonylamino)benzimidazole, 5-butyl-2-(methoxycarbonylamino)benzimidazole, 5-propoxy-2-(methoxycarbonylamino)benzimidazole, 5-ethoxy-2-ethoxycarbonyl-aminobenzimidazole, 5-propylthio-2-(methoxycarbonylamino)benzimidazole, 5-phenylthio-2-(methoxycarbonylamino)benzimidazole, 5-phenylsulphinyl-2-(methoxycarbonylamino)benzimidazole, 5-(2,4-dichlorophenoxy)-6-chloro-2-methylthiobenzimidazole, 6-chloro-5-(2,3-dichlorophenoxy)-2-methylthiobenzimidazole, 2-(4-thiazolyl)benzimidazole, and 5-isopropoxycarbonylamino-2-(4-thiazolyl)benzimidazole, however, data is provided only for albendazole (i.e., 5-propylthio-2-methoxycarbonyl-aminobenzimidazole).
Indeed, triclabendazole is the current drug of choice against mature and immature liver flukes. Not surprisingly, however, reports of parasite resistance are increasing. For example, Mottier et al., report that a population of resistant F. hepatica (Sligo) may use an altered influx/efflux mechanism to selectively decrease the amount of triclabendazole and triclabendazole sulfoxide but not albendazole. See Mottier et al., J. Parasitol., 92(6), 2006, pp. 1355-1360. McConville et al., report that juvenile triclabendazole-resistant F. hepatica are somewhat susceptible to compound alpha (i.e., 5-chloro-2-methylthio-6-(1-naphthyloxy)-1H-benzimidazole) via a tubulin-independent mechanism. See McConville et al., Parasitol. Res., (2007) 100:365-377. Further, Keiser et al., report the testing of artemether and OZ78 in triclabendazole-resistant F. hepatica, although at high concentrations. For a short review of triclabendazole resistance see Brennan et al., Experimental and Molecular Pathology, 82, (2007) pp. 104-109.
The resistance to triclabendazole and lack of effective substitutes creates a pressing need in the field for alternatives that exhibit low side effects and that do not contaminate the animals as a food source. Optimal compositions should further be efficacious, have a quick onset of activity, have a long duration of activity, and be safe to the animal recipients and their human owners.